They were 400 feet long and nearly 40 feet high and could carry a crew of 144. The submarines were designed to carry three “fold-up” bombers that could quickly be assembled.

Kerby said the main hull is sitting upright and is in good shape. The I-401 numbers are clearly visible on the sides, and the anti-aircraft guns are in almost perfect condition, he said.

An I-400 and I-401 were captured at sea a week after the Japanese surrendered in 1945. Their mission, which was never completed, reportedly was to use the aircraft to drop rats and insects infected with bubonic plague, cholera, typhus and other diseases on U.S. cities.

When the bacteriological bombs could not be prepared in time, the mission reportedly was changed to bomb the Panama Canal. Both submarines were ordered to sail to Pearl Harbor and were deliberately sunk later, partly because Russian scientists were demanding access to them.

The submarine found Thursday is the second Japanese vessel discovered off Oahu by the Hawaii Undersea Research Laboratory. In 2002, researchers found the wreckage of a much smaller Japanese sub that was sunk on Dec. 7, 1941, off Pearl Harbor.